High Current Amplifiers:
An amplifier designated as a high current amplifier is generally a class A/B amplifier which can drive a lower impedance load than the industry standard 2 ohms per channel. Some high current amplifiers can drive loads as low .5 ohms stereo or even .67 ohms mono (yikes!). This page will give a review of amplifier basics and will show some of the differences between standard and high current amplifiers.
Note:
The information below has links which will take you back to an earlier page from the site which will explain the technical terms involved. If you don't understand something, go back and read it (it doesn't cause any physical pain... really)
BACKGROUND INFORMATION:
1.The amplifier uses the power supplied by the battery or charging system.
2.The amp's switching power supply uses a transformer to boost the available voltage (from the battery/charging system).
3.The amount that the voltage is boosted is determined, in part, by the winding ratio of the
transformer.
4.The output of the transformer is a high
frequency
AC
voltage.
5.This AC voltage is rectified to a positive DC voltage (considered to be the positive 'rail' voltage) and a negative DC voltage (this is the negative rail voltage).
6.The rail voltages are both higher than ground and lower than ground (plus and minus voltages with respect to ground).
7.The audio output transistors act as electronic valves to deliver varying amounts of rail voltage to the
speaker.
8.The audio output transistors cannot deliver more than the voltage present on the rails.
9.The power that can be delivered to the speaker is determined by the amount of voltage that can be delivered to the speaker.
10.If the amplifier has plus and minus 30 volt rails (60 volts rail to rail), it can deliver, at most, 30 volts to the speaker at any given time. If we use the Ohm's law formula, P=E^2/R and we have a 4 ohm speaker:
P=30^2/4
P=900/4
P=225 watts peak, 112.5 watts
RMS
*Keep in mind that the next numbers don't take into account any inefficiencies or losses.
If we use a 2 ohm load and pretend that the rail voltage won't sag, we get:
P=900/2
P=450 watts peak, 225 RMS
The example(s) above show how lowering the impedance of a speaker load increases the output power.
11.
When you reduce the load from 4 ohms to 2 ohms, the current through the output transistors doubles (ignoring inefficiencies). The transistors have a safe operating area which, if exceeded, will likely lead to their demise. High current amplifiers have to reduce the rail voltage so that there won't be too much current flowing through the outputs when driving a low impedance load. You're probably thinking that they could simply use more transistors and they do... for their larger amplifiers.
Common Practice:
Many manufacturers try to get the most bang for the buck. If they want to build two amplifiers which will produce a given amount of power (let's say 600 watts), they don't have to build 2 completely different amplifiers. They can just change a few parts and have another amplifier (i.e. high current) to add to their product line (with virtually no added R&D or production cost). The difference between a 600 watt 2 ohm stable amp and a 600 watt high current amp may be only the transformer winding ratio. The example below shows the difference between 2 amplifiers and shows how they produce the same power into 2 different loads.
Test example:
High Current Amplifier:
No load rail voltage: +/ 36 volts
RMS voltage delivered into a 1 ohm load per channel: 19.6 volts
RMS power into each 1 ohm stereo load: 384 watts
Current draw at full power: 101 amps
Standard 2 Ohm Stable Amplifier:
No load rail voltage: +/ 47 volts
RMS voltage delivered into a 2 ohm load per channel: 27.7 volts
RMS power into each 2 ohm stereo load: 384 watts
Current draw at full power: 76 amps
You can see that the 2 ohm stable amp has a higher rail voltage and
therefore a higher output voltage. The HC amp has 36 volt rails the second
amp has 47 volt rails but they both produce the same amount or power.
Note:
From the previous example, you will see that the high current amplifier draws more current from the 12 volt source but produces the same power output. If the alternator could not maintain its target output voltage under the tougher load from the high current amplifier, the HC amplifier would not be able to produce as much power as the non high current amplifier.
OAQ (once asked question):
If you reduce the speaker's impedance from 2 ohms to 1 ohms, will the current draw double?
Answer:
Ignoring losses, the current draw would double because the current through the outputs (to the load) would double. In reality, the rail voltage would sag, so the speakers probably wouldn't get quite twice the voltage and therefore the current through the outputs would not quite be doubled but the losses in all of the components (transistors, copper, resistors, transformer) would be greater, so the current draw from the 12 volt source would still be at least double the 2 ohm current draw. In general, all else being equal, a high current amplifier will be less efficient than an amplifier optimized for a higher impedance load.
